Using the specific sulfide ion electrode in combination with shifts in the visible absorption spectra and electrophoretic separation of products of the reaction between radiosulfide and blood pigments, the mechanisms of binding or inactivation of hydrosulfide anion by various kinds of methemoglobins will be examined. Of particular interest are the methemoglobins generated in the reactions of sodium nitrite and potassium ferricyanide with oxyhemoglobin. However, hydroxylamine, phenylhydroxylamine, and perhaps other compounds appear to share with nitrite the capability of producing alterations in the hemoglobin molecule associated with an increased capacity to bind or inactivate sulfide. Ferricyanide-methemoglobin binds sulfide to its heme iron, but seems to lack the extra-heme sulfide binding sites of nitrite- methemoglobin. This property is probably related to the unique reaction mechanism which leaves iron tightly bound to the globin moiety and results in a quantitative release of the heme oxygen. No other compound tested appears to quantitatively release the heme oxygen and some produce a net oxygen consumption on reaction with hemoglobin. It is anticipated that these reactions can be studied by combining polarographic techniques with the classical manometric approaches. Eventually, it may be possible to establish a connection between consumption of the heme oxygen during conversion of oxyhemoglobin to methemoglobin, the extra-heme sulfide binding sites and a third phenomenon which we have called hemoglobin X. Preliminary evidence indicates that an unrecognized pigment may exist in the sequence methemoglobin, deoxyhemoglobin to oxyhemoglobin when the reaction is catalyzed by methylene blue. The significance of the transient existence of hemoglobin X is unknown. At the least it represents an important analytical artifact, but it may constitute a new pigment with an important physiological role.